Phenotypic plasticity affects an array of human characteristics, from height and weight to disease susceptibility. Yet the mechanisms by which environmental inputs are detected and then alter developmental programs are not well understood, and experimental models for studying this link are limited. This project addresses the fascinating problem of how both the environment and genetic variation influence the development of alternative phenotypes using an innovative model system, the pea aphid. This species offers an unparalleled opportunity to examine the role of nature and nurture in phenotype determination: it exhibits dramatically different winged and wingless morphs that are induced by environmental conditions in asexual females and by a single unidentified genetic locus in males. Thus, strikingly, two dimorphisms, each under different control mechanisms, exist within this single species. Here we propose a comprehensive investigation of the developmental and genetic basis of both dimorphisms, the first study of its kind, with a research plan that builds upon extensive preliminary data. In Aim 1 we will identify and characterize the wing polymorphism locus to illuminate how this single locus can cause dramatic differences in morphology in males. Our methods include association mapping using genome sequencing, gene expression level profiling across development using qRT-PCR, gene expression spatial profiling using in situ hybridization at targeted developmental stages, and mutant analysis using the CRISPR/Cas9 system. In Aim 2, we will study the hormonal basis of wing determination to ascertain how environmental cues are translated into phenotypic differences in females. We will test the role of a steroid hormone, ecdysone, in this process. We will measure ecdysone titer levels in induced mothers (winged offspring producers) versus uninduced mothers (wingless offspring producers), and experimentally manipulate their offspring phenotypes via injection of ecdysone. Further, we will measure expression levels of ecdysone-responsive genes via qRT-PCR in the embryos of mothers receiving or not receiving wing-inducing signals. In Aim 3, we will examine the extent to which there are shared mechanisms underlying the environmentally induced and genetically controlled dimorphisms, to establish how the two dimorphisms are related at the mechanistic level. We will approach this problem by looking for an effect of mutating the male polymorphism gene on the female alternative morphs and by measuring gene expression of ecdysone-responsive genes in winged and wingless male aphids via qRT-PCR. Overall, this research will result in a model of how both environmental and genetic factors influence similar alternative phenotypes. We anticipate that a major finding of this work will be the illumination of how environmental signals are bypassed when there is an evolutionary transition to genetic control of phenotypic variation.